1. Field of the Invention
The present invention relates generally to a radio-over-fiber (RoF) communications and in particular, to an RoF link apparatus for upstream/downstream wireless signal transmission in a mobile communication system using a Time Division Multiplexing (TDD) method.
2. Description of the Related Art
Typically, a mobile communication system uses repeaters to expand cell coverage thereof. Further, in underground or the buildings, where electronic wave cannot reach, optical repeaters using an optical link are widely used. The optical link is used, in one direction, to transmit a wireless signal to an optical repeater which is further used to transmit a wireless signal in an existing mobile communication system, e.g., a Code Division Multiple Access (CDMA) or a Wideband CDMA (WCDMA) system.
The mobile communication system uses a Frequency Division Duplexing (FDD) method using divided frequencies and a Time Division Duplexing (TDD) method using divided times as a duplexing method for distinguishing an uplink and a downlink for wireless signal transmission.
The CDMA and WCDMA systems mainly use the FDD method, and Wireless Broadband Internet (WiBro) and 4th Generation Mobile Communication (4G) systems, which are up-to-date mobile communication systems consider the TDD method.
The TDD method is considered as a method suitable for next-generation mobile communication systems using multiple antennas since an upstream/downstream transmission size varies freely and channel characteristics of upstream and downstream wireless signals are substantially the same. The TDD method has a different characteristic from the FDD method in that an upstream signal is transmitted in a predetermined time and a downstream signal is transmitted in the other time.
FIG. 1 is a block diagram of a conventional radio-over-fiber (RoF) link apparatus for upstream/downstream wireless signal transmission/reception in a TDD mobile communication system.
Referring to FIG. 1, the conventional RoF link apparatus includes a main donor 120 and a remote 130 connected to the main donor 120 via an optical fiber 140.
The main donor 120 existing in a Central Station (CS) is connected to an Access Point (AP) 110 via a Radio Frequency (RF) cable, electro-optic converts an RF signal received from the AP 110 to an optical signal and transmits the converted optical signal to the remote 130 of an optical repeater via the optical fiber 140, and opto-electric converts an optical signal received from the remote 130 to an RF signal and transmits the converted RF signal to the AP 110 via the RF cable. The AP 110, which is a base station of the mobile communication system, transmits data received from a Packet Access Router (PAR: not shown) to a terminal (not shown) in a wireless manner, and has a low-power RF/Intermediate Frequency (IF) module and controller function, an Orthogonal Frequency Division Multiplexing (OFDM)/TDD packet scheduling and channel multiplexing function, a Media Access Control (MAC) frame variable control function according to a service characteristic and a radio wave environment, a 50 Mbps-level high-speed traffic real-time control function, and a handover function.
The remote 130 of the optical repeater opto-electric converts an optical signal received from the main donor 120 to an RF signal and transmits the converted RF signal to a terminal (not shown) via an antenna, and electro-optic converts an RF signal received from the terminal to an optical signal and transmits the converted optical signal to the main donor 120 via the optical fiber 140.
Configurations of the main donor 120 and the remote 130 will now be described in more detail. The main donor 120 includes a Low Noise Amplifier (LNA) 121, an electro-optic converter (E/O) 122, a Wavelength Division Multiplexer (WDM) 123, an opto-electric converter (O/E) 124, and a High Power Amplifier (HPA) 125. The remote 130 includes a WDM 131, an O/E converter 132, a coupler 133, an HPA 134, a switch 135, an LNA 136, an E/O converter 137, and a switch timing signal generation circuit 138.
Although it is not shown, the main donor 120 can expand coverage of optical repeaters by connecting to a plurality of remotes via the optical fiber 140. To do this, the main donor 120 can include a signal divider (not shown) having multiple channels and a signal combiner (not shown), wherein the signal divider divides an RF signal received from the AP 110 and transmits the divided RF signals to corresponding LNA 121, and the signal combiner combines an RF signal received from the HPA 125 with outputs of other remotes 130 and transmits the combined RF signal to the AP 110 via the RF cable.
Each of the WDMs 123 and 131 is a device for allowing an optical fiber channel to be used as a plurality of communication paths by dividing the optical fiber channel into a plurality of channels using a wavelength of light, and can operate as a wavelength division multiplexer (MUX) for transmitting signals having a plurality of optical wavelengths by carrying the signals on a single optical fiber when an optical signal is transmitted or can operate as a wavelength division demultiplexer (DEMUX) for distributing signals having a plurality of optical wavelengths carried on a single optical fiber when an optical signal is received.
The E/Os 122 and 137 can be implemented using a laser diode, and the O/Es 124 and 132 can be implemented using a photo diode.
When the coupler 133 extracts a portion of an RF signal, the switch timing signal generation circuit 138 generates a switch timing signal for controlling the switch 135 by distinguishing downstream data and upstream data and provides the generated switch timing signal to the switch 135. An internal configuration of the switch timing signal generation circuit 138 will be described in more detail with reference to FIG. 2.
Data transmission procedures of an uplink and a downlink using the components of the main donor 120 and the remote 130 will now be described in detail. For the downlink, an RF signal transmitted via the RF cable from the AP 110, which is an upper layer, is input to the LNA 121 of the main donor 120.
The LNA 121 of the main donor 120 substantially cancels a noise component of the RF signal, amplifies a signal component of the RF signal, and outputs the substantially noise-cancelled and signal-amplified RF signal to the E/O 122. The E/O 122 converts the input RF signal to an optical signal and outputs the converted optical signal to the WDM 123. The WDM 123 transmits a plurality of optical signals input from the E/O 122 to the remote 130 via the optical fiber 140.
The WDM 131 of the remote 130, which has received an optical signal from the main donor 120, demultiplexes the optical signal and outputs the demultiplexed optical signals to the O/E 132. The O/E 132 converts the input optical signal to an RF signal and outputs the converted RF signal to the HPA 134.
The HPA 134 amplifies the input RF signal up to an effective power enough to transmit it in a wireless manner and outputs the amplified RF signal to the switch 135. The switch 135 emits the input RF signal to the terminal via the antenna.
For the uplink, when an RF signal is received from the terminal via the antenna of the remote 130, the LNA 136 substantially cancels a noise component of the RF signal, amplifies a signal component of the RF signal, and outputs the substantially noise-cancelled and signal-amplified RF signal to the E/O 137. The E/O 137 converts the input RF signal to an optical signal and outputs the converted optical signal to the WDM 131. The WDM 131 transmits the optical signals input from the E/O 137 to the main donor 120 via the optical fiber 140.
The WDM 123 of the main donor 120, which has received the optical signal from the remote 130, demultiplexes the optical signal into a plurality of optical signals and outputs the demultiplexed optical signals to the O/E 124. The O/E 124 converts the input optical signal into an RF signal and outputs the converted RF signal to the HPA 125.
The HPA 125 amplifies the input RF signal up to an effective power sufficient to transmit it to the AP 110 and transmits the amplified RF signal to the AP 110 via the RF cable.
The coupler 133 of the remote 130 extracts a portion of the RF signal transferred from the O/E 132 to the HPA 134 and outputs the extracted RF signal to the switch timing signal generation circuit 138. The switch timing signal generation circuit 138 generates a switch timing signal for RF signal transmission by analyzing the extracted RF signal and outputs the generated switch timing signal to the switch 135. In response to the input switch timing signal, if a downstream signal is input, the switch 135 emits the downstream signal to the terminal via the antenna, and if an upstream signal is input, the switch 135 cuts off a path connected to the HPA 134 and sets a path for providing the upstream signal to the LNA 136.
FIG. 2 is a block diagram of the switch timing signal generation circuit 138 illustrated in FIG. 1.
Referring to FIG. 2, the switch timing signal generation circuit 138 includes a divider 210, a level detector 220, a variable gain amplifier (VGA) 230, a log-scale amplifier 240, a pulse-shape generator 250, a comparator 260, a reference pulse-shape generator 270, a phase tuning circuit 280, and a timing controller 290 as internal components.
A process of generating a switch timing signal in the switch timing signal generation circuit 138 will now be described in detail.
After the coupler 133 extracts a portion of an RF signal and outputs the extracted RF signal to the divider 210, the divider 210 distributes the RF signal to the level detector 220 and the VGA 230. The level detector 220 measures a level of the RF signal and outputs the measured level to the VGA 230. The VGA 230 maintains an output signal to a constant level based on the measured level input from the level detector 220.
The log-scale amplifier 240 converts a signal variation input from the VGA 230 from a linear scale to a dB scale and outputs the scale-converted signal variation to the pulse-shape generator 250. The pulse-shape generator 250 generates a pulse-shape signal using the input signal and outputs the generated pulse-shape signal to the comparator 260.
The reference pulse-shape generator 270 generates a reference pulse-shape signal for determining a frame start position of the RF signal by correlating with the pulse-shape signal generated by the pulse-shape generator 250 and outputs the generated reference pulse-shape signal to the comparator 260.
The comparator 260 performs a correlation between the pulse-shape signal input from the pulse-shape generator 250 and a reference pulse-shape signal input from the reference pulse-shape generator 270. That is, the comparator 260 correlates the two signals and outputs a correlation result to the timing controller 290. The timing controller 290 determines the frame start position of the RF signal extracted by the coupler 133 by analyzing the correlation result input from the comparator 260 and calculates a start time of the downstream or upstream signal based on the determined frame start position.
The timing controller 290 generates a switch timing signal for controlling the switch 135 using the calculated start time information of the downstream or upstream signal and outputs the generated switch timing signal to the switch 135.
The phase tuning circuit 280 receives phase information of the pulse-shape signal generated by the pulse-shape generator 250 from the comparator 260 and tunes a phase of the reference pulse-shape signal.
When the switch timing signal generation circuit 138 generates a switch timing signal through the above-described process and outputs the generated switch timing signal to the switch 135, the switch timing signal controls the switch 135 by distinguishing an RF signal input to the switch 135 as either a downstream signal or an upstream signal. That is, it a downstream signal is input from the HPA 134 of the remote 130 to the switch 135, the switch timing signal controls the switch 135 to emit the downstream signal to the terminal via the antenna, and if an upstream signal is input from the terminal via the antenna, the switch timing signal controls the switch 135 to provide the upstream signal to the LNA 136 of the remote 130. Thus, the switch 135 can selectively provide a path of a downstream signal or an upstream signal by controlling the opening or closing thereof in response to the switching timing signal.
As described above the conventional RoF link apparatus used in a TDD mobile communication system uses a method of assigning a single wavelength to an upstream signal and second single wavelength to a downstream signal as in a conventional RoF link apparatus used in an FDD mobile communication system.
However, the conventional RoF link apparatus used to connect a wireless upstream/downstream signal to an optical repeater in a TDD mobile communication system transmits the upstream and downstream signals by assigning different wavelengths to them as in transmission of an FDD wireless upstream/downstream signal. That is, for signal transmission to a single optical repeater, two wavelengths must be used, and in order to identify the two wavelengths, the MUX/DEMUX of the WDM 123 or 131 must be used.
In order to use optical transmitters using different wavelengths, the two optical transmitters must easily identify the wavelengths used for upstream and downstream signals. In addition, when a plurality of optical repeater signals are transmitted using a single optical fiber, a WDM scheme is used, and due to this, 2×N wavelengths must be used in order to transmit signals to N optical repeaters, resulting in high cost.